1. Field of the Invention
This invention relates to a casing for a display device such as a fluorescent display device, and more particularly to a casing for a display device of the tipless type that an evacuation tube is not used.
2. Description of the Prior Art
A typical fluorescent display device is a display device which has been conventionally used. In the fluorescent display device, a casing in which electrodes such as an anode, a control electrode, a cathode and the like are arranged in an evacuation tube having a tip tube projecting outwardly from the casing through which the interior of the casing is evacuated to a high vacuum. Once the evacuation of the casing is carried out, the tip tube is sealed by melting to keep the casing at a high vacuum.
However, the conventional casing has a disadvantage of causing a display device to have a low space factor, because the tip tube remains projected outwardly from the casing after it is sealed. Also, the tip tube is formed of glass inferior in impact resistance, resulting in the fluorescent display device being inferior in durability.
In view of the foregoing, a casing for a fluorescent display device of the tipless type has been proposed which is constructed in a manner as shown in FIG. 1. In the casing shown in FIG. 1, a glass substrate A is formed with a through-hole B and a ceramic element D is bonded onto the inner surface of the through-hole B by means of frit glass C. The ceramic element D is formed with a through-hole E of a smaller diameter substantially concentrical with the through-hole B, and a metallized layer F is deposited on the inner side surface of the through-hole E and on the surface portion of the ceramic element D adjacent to the through-hole E and opposite to the through-hole B. The hermetic sealing of the casing is carried out by heating the vicinity of the through-hole E to melt a brazing material G filled in the through-hole E. After the casing is evacuated, material G is filled in the through-hole E.
However, the conventional casing of such construction has not been put into practice due to the following disadvantages.
One of the disadvantages is that the casing is hard to be manufactured and complicated in structure because it is required to form the ceramic element highly hard in working with the through-hole E and also it is necessary to fix the ceramic element D with respect to the through-hole B of the glass substrate A from the inside of the casing in a specific atmosphere.
Another disadvantage is that a vapor of metal generated from the brazing material G and gas generated from organic flux by heating remain in the casing not only to be reactively absorbed in an oxide cathode to cause sintering, to thereby hinder the electron discharging capacity of the cathode but to cause the decrease in vacuum within the casing and the contamination of the surface of a fluorescent layer to decrease display characteristics of the fluorescent display device, because the through-hole E of the substrate A is sealedly filled with the brazing material G by melting.